It has been known to dry agricultural feed materials like grass, lucerne, sugar beet slices, potatoes and other products, in order to preserve them, by using drying apparatus, especially drum-type drying apparatus. In such apparatus the drying air exits into the atmosphere saturated more or less with steam and at a temperature between about 100.degree. through 150.degree. C. There have been numerous prior attempts to make use of the exhaust air existing into the atmosphere and in order to reduce thereby the energy use of the drying apparatus. Such attempts ran aground so far on the high additional costs of the apparatus involved. Similar problems are involved also when pulp-like materials are being dried.